Lithium ion secondary batteries, which are a typical representation of non-aqueous electrolyte secondary batteries, have a high electromotive force and a high energy density, and therefore demand for the batteries is growing as a main power source for mobile communication devices and portable electronic devices. Most of the lithium ion secondary batteries currently available in the market employ, as a positive electrode active material, a lithium composite oxide (e.g., LixCoO2 (where x varies according to charge and discharge of the battery)) composed mainly of cobalt. However, because lithium composite oxides composed mainly of cobalt require, as a raw material, cobalt compounds which are costly, it is difficult to achieve cost reduction.
From the view point of achieving cost reduction, research and development has been conducted on various positive electrode active materials as an alternative to lithium cobalt oxide. Particularly, lithium composite oxides composed mainly of nickel (e.g., LixNiO2 (where x varies according to charge and discharge of the battery)) are being vigorously studied.
Besides cost reduction, it is also important to increase reliability of lithium ion secondary batteries. A lithium composite oxide such as LixCoO2 or LixNiO2 has Co4+ or Ni4+ which is in a state of a high valence having a high reactivity during charge. This facilitates a decomposition reaction of an electrolyte in which a lithium composite oxide participates in a high temperature environment, generating gas in the battery and degrading life characteristics. It is known that LixNiO2 has a higher reactivity with electrolytes in a charged state than LixCoO2.
In order to suppress the decomposition reaction of electrolytes, there is proposed to suppress the decomposition reaction of an electrolyte in which a lithium composite oxide participates by forming a coating made of a specific metal oxide on a surface of a positive electrode active material (Patent Documents 1 to 5). As a technique of improving cycle characteristics, the control of Li amount in a positive electrode active material in an end-of-charge/discharge state is also proposed (Patent Document 6).
Patent Document 1: Japanese Laid-Open Patent Publication No. Hei 9-35715
Patent Document 2: Japanese Laid-Open Patent Publication No. Hei 11-317230
Patent Document 3: Japanese Laid-Open Patent Publication No. Hei 11-16566
Patent Document 4: Japanese Laid-Open Patent Publication No. 2001-196063
Patent Document 5: Japanese Laid-Open Patent Publication No. 2003-173775
Patent Document 6: Japanese Laid-Open Patent Publication No. Hei 5-290890